make: add submodule support

Makefile.base

@@ -22,6 +22,23 @@ ${DIRS:%=ALL--%}:
 ${DIRS:%=CLEAN--%}:
 	$(QQ)"$(MAKE)" -C ${@:CLEAN--%=%} clean
 
+## submodules
+ifeq (1, $(SUBMODULES))
+    # don't use *.c as SRC if SRC is empty (e.g., no module selected)
+    NO_AUTO_SRC := 1
+
+    # allow different submodule basename (e.g., MODULE=cpu_periph_common, but match just periph_%)
+    BASE_MODULE ?= $(MODULE)
+
+    # for each $(BASE_MODULE)_<name> in USEMODULE, add <name>.c to SRC
+    SRC += $(patsubst $(BASE_MODULE)_%,%.c,$(filter $(BASE_MODULE)_%,$(USEMODULE)))
+
+    # don't fail if a selected *.c file does not exist
+    ifeq (1, $(SUBMODULES_NOFORCE))
+        SRC := $(filter $(SRC), $(wildcard *.c))
+    endif
+endif
+
 ifeq ($(strip $(SRC))$(NO_AUTO_SRC),)
     SRC := $(filter-out $(SRC_NOLTO), $(wildcard *.c))
 endif